Polymer electrolyte fuel cells are characterized by using a protonic conductive polymer electrolyte membrane, and are devices that provide an electromotive force by electrochemically reacting a fuel gas such as hydrogen with an oxidizing gas such as oxygen. The polymer electrolyte fuel cells can be utilized as private power generators and power generators for mobile bodies such as automobiles.
Such a polymer electrolyte fuel cell has a polymer electrolyte membrane which selectively conducts hydrogen ions (protons). The fuel cell has two sets of gas diffusion electrodes and has a structure as described below. The gas diffusion electrode has a catalyst layer which contains, as a main component, a carbon powder which supports a noble metal-based catalyst, and has a gas diffusion electrode substrate. Each of the gas diffusion electrodes is joined to the surface of a polymer electrolyte membrane with the catalyst layer facing inward.
An assembly composed of such a polymer electrolyte membrane and two sets of such gas diffusion electrodes is referred to as a membrane electrode assembly (MEA). On both outer sides of the MEA, separators are installed in which gas flow paths are formed in order to feed a fuel gas and an oxidizing gas and to discharge produced gases and excessive gases.
A gas diffusion electrode substrate needs mechanical strength because the gas diffusion electrode substrate is fastened by a load of several megapascals by a separator in order to reduce the electric contact resistance and suppress the leakage of a fuel gas or an oxidizing gas fed from the separator to the outside a fuel cell.
Since a gas diffusion electrode substrate needs to mainly have the following three functions, the gas diffusion electrode substrate is usually a porous electrode substrate having a porous structure. A first function required for the gas diffusion electrode substrate is the function of uniformly feeding a fuel gas or an oxidizing gas, from a gas flow path formed in a separator which is arranged outer side of the gas diffusion electrode substrate, to a noble metal-based catalyst in a catalyst layer. A second function is a function of discharging water produced by a reaction in the catalyst layer. A third function is a function of conducting electrons necessary for a reaction in the catalyst layer or electrons produced by a reaction in the catalyst layer to the separator. What is considered to be an effective way to realize these functions is to employ a gas diffusion electrode substrate that generally uses a carbonaceous material.
Conventionally, in order to increase mechanical strength of the substrate, short carbon fibers were formed to a paper and bound one another by using organic polymers, and then this paper is firing at a high temperature to carbonize the organic polymers and to produce a porous electrode substrate which is composed of carbon/carbon composites in paper shape. However, the production process is complicated and a problem thereof is high cost. Although, in order to reduce the cost, a porous electrode substrate is proposed which is obtained by forming a paper from oxidized short fibers, and thereafter firing the paper at a high temperature to carbonize the oxidized short fibers, since the oxidized short fibers shrink during firing, problems of the electrode substrate are the dimensional stability thereof and a large undulation (the state of the sheet cross-section being waved or the state of that being warped).
Patent Literature 1 discloses a porous carbon electrode substrate for a fuel cell having features that include a thickness of 0.05 to 0.5 mm and a bulk density of 0.3 to 0.8 g/cm3, and a bending strength of 10 MPa or higher and a deflection in bending of 1.5 mm or more in a 3-point bending test under the conditions of a strain rate of 10 mm/min, a distance between fulcrums of 2 cm and a test piece width of 1 cm. However, although the porous electrode substrate exhibits high mechanical strength, small undulation, sufficient gas permeability and sufficient electroconductivity, the problem thereof is high production cost.
Patent Literature 2 discloses a carbon fiber sheet having a thickness of 0.15 to 1.0 mm, a bulk density of 0.15 to 0.45 g/cm3, a carbon fiber content of 95% by mass or more, a compression deformation ratio of 10 to 35%, an electric resistivity of 6 mΩ or lower, and a degree of drape of 5 to 70 g. Although this method for producing the carbon fiber sheet can be at a low cost, since shrinkage during firing is large, problems that occur in the resulting porous electrode substrate include a large unevenness in the thickness and large undulation.
Patent Literature 3 discloses a porous electrode substrate which is obtained by carbonizing a sheet composed of carbon fibers and acrylic pulp fibers. Although the porous electrode substrate can be produced at a low cost, since there is little entanglement between the carbon fibers and the acrylic pulp fibers during the process of forming the sheet, handling the porous electrode substrate is difficult. Comparing the acrylic pulp fibers with common fibrous materials, since the polymer exhibits almost no molecular orientation, the carbonization ratio during carbonization is low; thus in order to raise the handleability, much of the acrylic pulp fiber needs to be added.